Dopamine is a neurotransmitter in the brain. Since this discovery, made in the 1950s, the function of dopamine in the brain has been intensely explored. To date, it is well established that dopamine is essential in several aspects of brain function including motor, cognitive, sensory, emotional and autonomous (e.g. regulation of appetite, body temperature, sleep) functions. Thus, modulation of dopaminergic function may be beneficial in the treatment of a wide range of disorders affecting brain functions. In fact, both neurologic and psychiatric disorders are treated with medications based on interactions with dopamine systems and dopamine receptors in the brain.
Drugs that act, directly or indirectly, at central dopamine receptors are commonly used in the treatment of neurologic and psychiatric disorders, e.g. Parkinson's disease and schizophrenia. Currently available dopaminergic pharmaceuticals have severe side effects, such as extrapyramidal side effects and tardive dyskinesia in dopaminergic antagonists used as antipsychotic agents, and dyskinesias and psychoses in dopaminergic agonists used as anti-Parkinson's agents. Therapeutic effects are unsatisfactory in many respects. To improve efficacy and reduce side effects of dopaminergic pharmaceuticals, novel dopamine receptor ligands with selectivity at specific dopamine receptor sub-types or regional selectivity are sought for. In this context, also partial dopamine receptor agonists, i.e. dopamine receptor ligands with some but not full intrinsic activity at dopamine receptors, are being developed to achieve an optimal degree of stimulation at dopamine receptors, avoiding excessive dopamine receptor blockade or excessive stimulation.
Compounds belonging to the class of substituted 4-(phenyl-N-alkyl)-piperazine and substituted 4-(phenyl-N-alkyl)-piperidines have been previously reported. Among these compounds, some are inactive in the CNS, some display serotonergic or mixed serotonergic/dopaminergic pharmacological profiles while some are full or partial dopamine receptor agonists or antagonists with high affinity for dopamine receptors.
A number of 4-phenylpiperazines and 4-phenylpiperidine derivatives are known and described, for example Costall et al. European J. Pharm. 31, 94, (1975), Mewshaw et al. Bioorg. Med. Chem. Lett., 8, 295, (1998). The reported compounds are substituted 4-phenylpiperazine's, most of them being 2-, 3- or 4-OH phenyl substituted and displaying DA autoreceptor agonist properties.
Fuller R. W. et al, J. Pharmacol. Exp. Therapeut. 218, 636, (1981) disclose substituted piperazines (e.g. 1-(m-trifluoromethylphenyl)piperazine) which reportedly act as serotonin agonists and inhibit serotonin uptake. Fuller R. W. et al, Res. Commun. Chem. Pathol. Pharmacol. 17, 551, (1977) disclose the comparative effects on the 3,4-dihydroxyphenylacetic acid and Res. Commun. Chem. Pathol. Pharmacol. 29, 201, (1980) disclose the comparative effects on the 5-hydroxyindole acetic acid concentration in rat brain by 1-(p-chlorophenol)-piperazine.
Boissier J. et al Chem Abstr. 61:10691c, disclose disubstituted piperazines. The compounds are reportedly adrenolytics, antihypertensives, potentiators of barbiturates, and depressants of the central nervous system.
A number of different substituted piperazines have been published as ligands at 5-HT1A receptors, for example Glennon R. A. et al J. Med. Chem., 31, 1968, (1988), van Steen B. J., J. Med. Chem., 36, 2751, (1993), Mokrosz, J. et al, Arch. Pharm. (Weinheim) 328, 143-148 (1995), and Dukat M.-L., J. Med. Chem., 39, 4017, (1996). Glennon R. A. discloses, in international patent applications WO 93/00313 and WO 91/09594 various amines, among them substituted piperazines, as sigma receptor ligands. Clinical studies investigating the properties of sigma receptor ligands in schizophrenic patients have not generated evidence of antipsychotic activity, or activity in any other CNS disorder. Two of the most extensively studied selective sigma receptor antagonists, BW234U (rimcazole) and BMY14802, have both failed in clinical studies in schizophrenic patients (Borison et al, 1991, Psychopharmacol Bull 27(2): 103-106; Gewirtz et al, 1994, Neuropsychopharmacology 10:37-40).
Further, WO 93/04684 and GB 2027703 also describe specific substituted piperazines useful in the treatment of CNS disorders.